Reading a book, and i dont mean a comic book. Grab a nice piece of literature with an extensive vocabulary. Reading helps you with learning new fancy words as well as getting you to think. Pick up a non-fiction book, relax,and enjoy!
Exercise. Spending thirty minutes to an hour doing some quality exercise can make you feel good (endorphins) as well as help you relax; improve your sleep and brain function.
meditating is a good way to lower everything in your body i lowered my heart rate to 35 BPM but i dont know how to increase brain activity
Board games, marble games, those really tend to get your brain in work without you having to do much of anything.
Cross word puzzles, instrumental music, reading/writing,etc